Polycatechol‑Modified Fe₃O₄ Magnetic Nanoparticles: A Highly Selective, Magnetically Recoverable Adsorbent for Cationic Dye Removal

Abstract

We report a facile coprecipitation route to synthesize polycatechol‑modified Fe₃O₄ magnetic nanoparticles (Fe₃O₄/PCC MNPs). The PCC layer confers a negatively charged, oxygen‑rich surface that binds cationic dyes through electrostatic attraction, while preserving superparamagnetic behavior for easy separation. Adsorption capacities for methylene blue (60.06 mg g⁻¹), cationic turquoise blue GB (70.97 mg g⁻¹), malachite green (66.84 mg g⁻¹), crystal violet (66.01 mg g⁻¹) and cationic pink FG (50.27 mg g⁻¹) were achieved. Kinetic data fit a pseudo‑second‑order model (R² > 0.997), and Langmuir isotherms described the equilibrium behavior. The material retains 70 % of its initial capacity after five adsorption–desorption cycles using ethanol at pH 4, underscoring its potential for industrial wastewater treatment.

Background

Dye effluents from textiles, paper, printing and other industries represent a major environmental threat due to their persistence, toxicity and vivid coloration. Conventional treatment methods—photocatalysis, coagulation, electrochemical oxidation, membrane filtration, biological degradation—often suffer from high cost, limited efficiency or secondary pollution. Adsorption stands out for its simplicity, cost‑effectiveness and high removal efficiency. Activated carbon and natural clays have been widely used, yet their magnetic recoverability is limited. Magnetic nanoparticles (MNPs), especially Fe₃O₄, combine high surface area, superparamagnetism and low cost, but unmodified particles tend to aggregate, reducing accessibility to adsorbates. Surface functionalization therefore becomes critical. Polycatechol, a polymer formed by Fe(III)-catalyzed catechol polymerization, offers abundant phenolic groups that bind metal oxides strongly and present a negatively charged, hydrophilic surface. To date, no study has exploited polycatechol‐modified Fe₃O₄ MNPs for dye adsorption.

Methods

Materials

FeCl₃·6H₂O, FeSO₄·7H₂O, NH₄OH, catechol and analytical‑grade dyes (methylene blue, cationic turquoise blue GB, malachite green, crystal violet, cationic pink FG) were obtained from Chuandong Chemical Inc. All reagents were used without further purification.

Preparation and Characterization of Fe₃O₄/PCC MNPs

Fe₃O₄/PCC MNPs were synthesized by adding 10 mmol FeCl₃·6H₂O and 5 mmol FeSO₄·7H₂O to 75 mL deionized water, followed by 75 mL 20 mM catechol (pH 2.87). Catechol polymerizes under Fe³⁺ catalysis, generating a black precipitate that serves as a template for Fe₃O₄ nucleation. After 30 min stirring, 100 mL 3.3 M NH₄OH was added, and the mixture aged for 120 min under vigorous stirring. The resulting black powder was magnetically collected, washed to neutral pH, and dried at 50 °C. For comparison, bare Fe₃O₄ MNPs were prepared identically without catechol.

Magnetic hysteresis was measured at 298 K with a Quantum Design MPMS XL‑7. Thermogravimetric analysis (TGA) was performed under N₂ at 5 °C min⁻¹. Zeta potentials across pH 3–10 were determined with a Malvern Zetasizer.

Batch Adsorption Experiments

Isotherms: 25 mg Fe₃O₄/PCC MNPs in 25 mL dye solutions (0.02–0.4 mM) were shaken at 180 rpm, 30 °C until equilibrium. Supernatants were analyzed by UV‑vis at each dye’s λ_max. Kinetics: 100 mg MNPs in 100 mL 0.1 mM dye solutions were sampled every few minutes up to 300 min. pH and temperature studies varied pH 3–9 and 30–45 °C, respectively. Co‑ion effects were examined with NaCl, MgSO₄, FeCl₃ (0.1–0.5 mM). Regeneration involved ethanol desorption at pH 4 for 12 h, followed by water washing. Adsorption capacity (q_e) was calculated by q_e = (C_i–C_e)V/M_s.

Results and Discussion

Characterization of Fe₃O₄/PCC MNPs

Figure 2a shows that Fe₃O₄/PCC MNPs exhibit a saturation magnetization of 53.5 emu g⁻¹, higher than bare Fe₃O₄ (49.6 emu g⁻¹), confirming the preservation of superparamagnetism. TGA (Fig. 2b) reveals a 5.2 % weight loss below 150 °C (adsorbed water), 9.4 % loss (150–400 °C) from oxygen‑bearing groups, 6.8 % (400–800 °C) from carbon combustion, and a slight 2.3 % gain above 800 °C (Fe₃O₄ → γ‑Fe₂O₃ oxidation). Zeta potential (Fig. 2c) shows an isoelectric point of 4.2 for bare Fe₃O₄ and a consistently negative surface for Fe₃O₄/PCC from pH 3 to 10, owing to phenolic hydroxyl groups. The enhanced negative charge stabilizes the particles against aggregation.

Selective Adsorption

Fe₃O₄/PCC MNPs preferentially adsorb cationic dyes, achieving 75.7 % removal of methylene blue, while anionic methylene orange and phenol are removed by only 10.9 % and 1.5 % (Fig. 3). Electrostatic attraction between the negatively charged surface and cationic dyes is the dominant mechanism.

Adsorption Kinetics

All five cationic dyes follow pseudo‑second‑order kinetics (R² > 0.997), with rate constants k_ad of 0.043–0.057 g mg⁻¹ mL⁻¹ (MB → FG). Pseudo‑first‑order fits are poor. The kinetic behavior confirms chemisorption as the rate‑determining step.

Isotherm Analysis

Langmuir fits outperform Freundlich for all dyes (Fig. 5). The maximum capacities q_m are: MB 60.06 mg g⁻¹, GB 70.97 mg g⁻¹, MG 66.84 mg g⁻¹, CV 66.01 mg g⁻¹, FG 50.27 mg g⁻¹. These values surpass many reported Fe₃O₄‑based adsorbents, demonstrating the advantage of polycatechol functionalization.

Effect of Temperature

Adsorption of MB increases with temperature (30–45 °C) to 84 % at 45 °C, indicating an endothermic process. GB and CV show reduced uptake at higher temperatures, suggesting exothermic, physical adsorption. Temperature has negligible influence on MG and FG, reflecting their distinct structural interactions with the MNP surface.

Effect of pH

Removal efficiency rises from pH 3 to 9 for all cationic dyes (Fig. 7), consistent with the increased negative surface charge of Fe₃O₄/PCC. Optimal adsorption occurs near neutral to slightly alkaline pH.

Co‑Ion Interference

Competitive cations (Na⁺, Mg²⁺, Fe³⁺) significantly suppress MB adsorption, with Fe³⁺ reducing uptake from 63 % to 20 % as its concentration rises from 0.1 mM to 0.5 mM (Fig. 8). This highlights the importance of electrostatic competition in real effluents.

Regeneration and Reusability

After ethanol desorption at pH 4 and water washing, Fe₃O₄/PCC MNPs retain 70 % of their initial capacity after five cycles. By the sixth cycle, removal efficiencies drop to 27–43 % for the five dyes, yet magnetic separation remains efficient, indicating good economic feasibility.

Conclusion

Polycatechol‑modified Fe₃O₄ magnetic nanoparticles exhibit high‑capacity, selective adsorption of cationic dyes, driven primarily by electrostatic interactions. Their superparamagnetic nature facilitates rapid magnetic recovery, and they can be regenerated with minimal performance loss. These properties make Fe₃O₄/PCC MNPs a promising candidate for treating dye‑contaminated industrial wastewater.